In "The Canterbury Tales", why did the corruption of the Church happen?
Does not power corrupt? In medieval times the kings and nobility had political power while the clergy had all spiritual authority. In addition, many of the nobility's second and third sons became priests because they did not stand to inherit any land or wealth directly from their father (this was the Norman tradition). And, because their motivation for entering the Church was not from a spiritual calling, they were more receptive to the material gain to be made in positions such as archbishop or cardinal.
This hierarchial organization of the Church and its dominance of education gave rise to abuse of the ignorant peasants. One flagrant example of this was the selling of indulgences and pardons for sins. Since the hierarchy of the church desired resplendent Cathedrals, the selling of these indulgences in order to pay for stained glass windows and magnificent architecture as well as maintaining the life style of the clergy (the Chaucer's Monk, tempted by money and materialism has his robes lined with fur) was out of control. Clergy who were not materialistic and corrupt protested, among them, of course, was Martin Luther.